Field of Invention
The present invention relates to optical modules for light projection and in particular, to a miniature optical light source and module for generating passive and dynamic structured light patterns using arrays, including addressable arrays of surface emitting light sources such as, VCSELs and/or RC-LEDs for applications such as, handheld devices for three dimensional (3-D) imaging, gesture recognition and other applications where compact illumination sources are required.
Background Art
Structured illumination is emerging as an important method for 3-D imaging of a objects in a scene (synonymously a scene hereinafter) in applications that vary from surveillance, gesture recognition, video gaming, Computer Aided Manufacturing (CAM), printing, shop-floor safety, etc., just to name a few. Structured illumination results in unique illumination patterns projected in different regions of the scene and a 3-D image of the scene is obtained by one of several methods known in the art. Most commonly used methods for 3-D imaging are triangulation, time of flight, stereoscopic imaging and often a suitable combination of these methods to get more accurate depth information for the 3-D image. It is therefore important that a device is capable of imaging in more than one way.
In structured light illumination a region of interest or a scene, is illuminated with pattern(s) having specific periodic or random features (microstructure hereinafter) and an image of the scene is obtained. Depending upon the distance of the scene or objects in the scene (from the structured light illumination source) the microstructure is distorted in a recorded image of the scene. A composite image may be generated by overlapping the recorded images to perform suitable analysis in relation to the projected structured illumination to estimate distance/depth information of the objects in the region. In triangulation or time of flight imaging methods, a camera is placed off axis to record the composite image of the structured illumination pattern reflected off of different objects in the scene. This basic methodology is adapted in many other 3-D imaging apparatus known in the art. In stereoscopic imaging at least two cameras are used to view the object using structured light illumination and the three dimensional aspect is generated by proper algorithms in much the same way as a pair of human eyes create a three dimensional visual effect.
In general, structured illumination apparatus comprises three parts—a light source, a pattern generator that generates a structured illumination pattern, and a projection apparatus which may include a single, or a combination of different optical elements for example, refractive or reflective elements (simple or a compound lens, one or more reflector, etc.). The light source includes one or more light emitters for example, a semiconductor light emitter such as, an edge-emitting Light Emitting Diode (LED), surface emitting LED or Resonant Cavity LED (RC-LED) or Vertical Cavity Surface Emitting Laser (VCSEL) that may be configured in arrays to achieve higher illumination intensity. The pattern generator may be an opaque pattern mask, or a more complex mask, such as a diffractive optical element (DOE), a hologram, or a combination of these methods known in the art. Often times, the projection apparatus includes the structured illumination pattern generator.
Use of a separate pattern generator such as a mask or DOE, results in significant light loss affecting brightness and resolution of the structured illumination pattern, particularly at the edges of the illuminated area. In another approach structured illumination patterns are generated and projected by translating, scanning or sweeping a VCSEL array source and the reflected light from an object is tracked synchronously with the translator or scanner device. However, scanning apparatus may limit the speed of imaging, may not be easily portable, and also may be less cost effective for volume applications.
In a different approach an array of semiconductor surface emitting source, such as surface emitting LEDs or RC-LEDs or VCSELs is used as a light source as well as repetitive or a random structured light pattern generator. Devices using an array of light sources are described in the United States Patent Application Publication No. 2013/0044187 by Hammes et al. published on Feb. 21, 2013, and United States Patent Application Publication No. 2012/0293625 by Schneider et al. published on Nov. 22, 2012, where a very large number of individually addressable VCSELs are used to generate random structured illumination patterns.
In a different approach described in the International Patent Application Publication No. WO 2014/083485 by Moench et al., published on Jun. 5, 2014, emission from several laser arrays, each one comprising an irregular distribution of emission areas are superimposed to project a desired pattern on a plane. In another approach disclosed in the U.S. Pat. No. 9,048,633 issued to Grönenborn on Jun. 2, 2015, and United States Patent Application Publication No. 2015/0108371 by Grönenborn et al. published on Apr. 23, 2015, a desired intensity distribution on a working plane is generated by VCSEL devices having differently shaped apertures to project alternate beam shapes.
One limitation of using an array of conventional surface emitting LEDs, RC-LEDs and VCSELs are the low emission power. As disclosed in the United States Patent Application Publication No. 2013/0044187 by Hammes et al. published on Feb. 21, 2013, a large number of devices are needed in an array to generate sufficient optical power. Furthermore, the resolution of the structured light pattern is limited by the pitch (distance between the adjacent devices) of the light emitter array, which in turn impacts the accuracy of depth information in 3-D measurement. For example, the smallest size of the typical state of the art LED or VCSEL device is 5 μm and the minimum spacing of devices in an array is about 15 μm, and more typically about 25 μm. Thus about 100,000 VCSEL devices may be accommodated in a 5×5 mm chip.
Structured light illumination may be generated in regular or irregular shapes and may include regular or irregular patterns. Moreover, different structured light patterns may be generated using the same apparatus. Some prior art methods include uniform illumination as well as structured light pattern illumination options in the same apparatus by using a switchable filter. Different projection methods, either using one step or two steps to cover a larger area or to reach a larger distance (from the light source) are known in the art to avoid image distortion due to diffraction and lens aberrations. Different prior art methods of structured light pattern generation and corresponding detection, are summarized in the ‘Background Art’ section of the U.S. patent application Ser. No. 14/848,791 filed by Seurin et al. on Sep. 9, 2015, the parent of this application. That description is being incorporated by reference herein.
The U.S. patent application Ser. No. 14/848,791 filed by Seurin et al. on Sep. 9, 2015, also discloses surface emitting sources that generate structured light illumination patterns including different microstructures as well as shapes in the same apparatus. That description is being incorporated by reference herein. More advanced VCSEL devices and arrays, and particularly those that have all electrical contacts are on one surface (top or bottom) of the device is a key to make them very compatible with electronics surface mount assembly technology and also highly suitable for high volume production processes. Examples of surface mountable VCSEL array are described in the U.S. Pat. No. 8,675,706 issued to Seurin et al. on Mar. 18, 2014, and in the United States Patent Application Publication No. 2013/0163627 by Seurin et al. published on Jun. 27, 2013, both co-owned by Princeton Optronics Inc. Mercerville, N.J., the Assignee of this application as well. Contents of the above mentioned patent and publication are being incorporated by reference in its entirety.
Another advancement in VCSEL design using three reflectors allows to increases the resonant cavity length enabling operation in a low order mode at higher power is described in the U.S. Pat. No. 9,268,012 issued to Ghosh et al. on Feb. 23, 2016, co-owned by Princeton Optronics Inc. Mercerville, N.J., the Assignee of this application as well. Three reflector VCSELs has other advantages of the surface mountable design disclosed in the U.S. Pat. No. 8,675,706 and are equally suited for constructing high power structured light illumination source. A further advancement in VCSEL design to obtain high power and brightness is disclosed in the U.S. patent application Ser. No. 14/700,010 by Wang et al. filed on Apr. 29, 2015, includes a gain region with multiple gain segments to boost output power. While the design may be adapted for a conventional two reflector VCSEL, best results are achieved in a three reflector VCSEL. Advantageously, the advanced VCSEL design disclosed therein also operates in a single and preferably linear polarization mode.
Additional optical elements such as microlens array are known to improve brightness and collimation of RC-LED and VCSEL array output beams. A microlens array may be placed externally at the focal distance or near the focal distance from the VCSEL elements. In general, each microlens array element is registered and aligned with the output beam axis of a corresponding VCSEL array to produce an array of coaxial collimated beams. The microlenses reduce the divergence of the beams and propagate highly directional beams to produce an array of high intensity spots that can be used as a structured illumination pattern in the region of interest. Use of microlens array to image the VCSEL aperture on to a distant plane to generate a desired intensity profile is described in the U.S. Pat. No. 9,048,633 issued to Grönenborn on Jun. 2, 2015. A desired intensity profile is generated by configuring an array of VCSEL arrays, each array including VCSELs with differently shaped apertures. In the United States Patent Application Publication No. 2015/0108371 by Grönenborn et al. published on Apr. 23, 2015, to a desired intensity distribution on a working plane is generated by allowing the microlens to be positioned offset from the center of the emission area.
There are several methods for fabricating and assembling the microlens array with the VCSEL or RC-LED array. A separate microlens array can be aligned and bonded in front of the VCSEL array as described in the U.S. Pat. No. 8,675,706 by Seurin et al. issued on Mar. 18, 2014, and the U.S. Pat. No. 9,268,012 issued to Ghosh et al. on Feb. 23, 2016. Alternatively, a microlens array may be constructed aligned with each element of the VCSEL (or RC-LED) array in an integrated fashion, particularly for an array where laser beam exits from the substrate as disclosed in the U.S. Pat. No. 6,888,871 issued to Zhang et al. on May 3, 2005. Contents of all the above mentioned patents, co-owned by Princeton Optronics Inc. Mercerville, N.J., also the Assignee of this application, are being incorporated by reference in entirety.
While the above mentioned methods to incorporate microlens array with the VCSEL array work well for relatively large illumination assemblies they have limitations when designing a miniature illuminator for handheld compact devices for 3-D imaging, gesture recognition, tablets, smart phones and laptop computers where the illuminator assembly is accommodated within a few millimeter thickness of the portable devices. Small apertures of imaging lens or microlens would block some of the emission from the VCSELs.
In this invention a miniature structured light illuminator and an illuminator module is provided using surface emitting light sources constructed to have different microstructure that may be used either alone or superimposed to generate different structured light patterns. Furthermore, surface emitting light source array is integrated with a microlens array designed to allow majority of the emission beam to be directed through small optical components and projected into a region of interest with minimum loss of intensity and distortion to the optimum structured illumination pattern. By suitably selecting a microlens array design to include different types of microlens, a wide range of convergent or divergent structured illumination pattern of different shapes and sizes may be generated to suit different applications within the general principles of this invention.